1. Field of the Invention
This invention relates to an illumination system, and more particularly to an illumination system for EUV lithography with wavelengths less than 193 nm.
2. Description of the Prior Art
To reduce the structural widths for electronic components, especially in the submicron range, it is necessary to reduce the wavelength of the light used for microlithography. For example, lithography with soft x-rays is conceivable with wavelengths smaller than 193 nm. U.S. Pat. No. 5,339,346 disclosed an arrangement for exposing a wafer with such radiation. An illumination system for soft x-rays, so-called EUV radiation, is shown in U.S. Pat. No. 5,737,137, in which illumination of a mask or a reticle to be exposed is produced using three spherical mirrors.
Field mirrors that show good uniformity of output of an exposure beam at a wafer in a lithographic system have been disclosed in U.S. Pat. No. 5,142,561. The exposure systems described therein concern the contact exposure of a wafer through a mask with high-energy x-rays of 800 to 1800 eV.
EUV illumination systems for EUV sources have been disclosed in EP 99 106 348.8 (U.S. application Ser. No. 09/305017) and PCT/EP99/02999. These illumination systems are adapted to synchrotron, wiggler, undulator, Pinch-Plasma or Laser-Produced-Plasma sources.
Scanning uniformity is a problem of the aforementioned scanning exposure systems in illuminating a slit, particularly a curved slit. For example, the scanning energy obtained as a line integral over the intensity distribution along the scan path in a reticle or wafer plane may increase toward the field edge despite homogeneous illumination intensity because of the longer scan path at the field edge for a curved slit. However, scanning energy and with it scanning uniformity may also be affected by other influences, for example coating or vignetting effects are possible. The curved slit is typically represented by a segment of a ring field, which is also called an arc shaped field. The arc shaped field can be described by the width delta r, a mean Radius R0 and the angular range 2·α0. For example, the rise of the scanning energy for a typical arc shaped field with a mean radius of R=100 mm and an angular range of
2·α0=60° is 15%.